Separation of hydrocarbons



Patented May 26, 1953 SEPARATION OF HYDROCARBONS Orville L. Polly and Clyde H. 0. Berg, Long Beach,

Calif., assignors to Union Oil Company of California, Los Angeles, Calif., a corporation of California No Drawing. Application July 30, 1948, L 7 Serial No. 41,731

9 Claims.

This invention relates to the separation of hydrocarbons, and in particular concerns an improved solvent extraction process for the separation of gaseous hydrocarbon mixtures.

In the commercial production of hydrocarbons by cracking or other operations the desired hydrocarbon products are almost invariably obtained in admixture with substantial quantities of other closely related hydrocarbons. Separation of such mixtures by ordinary distillation methods can seldom be accomplished satisfactorily since the various components of the mixtures usually have boiling points lying very closely together. Thus, for example, it is difiicult to separate butadiene from mixtures thereof with other hydrocarbons having four carbon atoms, 1. e., butanes and butenes, since there is only about 10 C. difference between the boiling points of the lowest-boiling and highest-boiling of these materials. Of the several known general methods for separating materials having almost equal boiling points, that of selective extraction by means of a liquid solvent is fundamentally practical by reason of its economy and ease of operation. The efficiency of any solvent extraction process, however, depends on the selectivity of the solvent employed.

It is an object of the present invention to provide an improved solvent extraction process for the separation of gaseous mixtures of hydrocarbons.

Another object is to provide a hydrocarbon separation process in which is employed novel selective solvents having a high degree of selectivity.

A further object is to provide an improved method for separatin mixtures of hydrocarbons having four carbon atoms.

Other objects will be apparent from the following detailed description of the invention and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.

We have now found that the above objects and attendant advantages may be realized in a process whereby gaseous hydrocarbon mixtures are subjected to selective solvent extraction with a mixed solvent comprising formamide. More particularly, we have found that liquid mixtures comprising formamide and ring-saturated heterocyclic compounds containing two hetero atoms selected from the class consisting of oxygen, sulfur, and nitrogen have a high degree of solvent selectivity for hydrocarbons, and that mixed hydrocarbons may accordingly be separated advan- 2 tageously by extracting them with a mixed solvent of such type.

The solvent extraction process of the present invention is particularly applicable to the separation of feed stocks consisting of mixtures of hydrocarbons having the same number of carbon atoms but having different degrees of unsatura tion, for example, mixtures of members of a horizontal group in the following table:

Degree of Unsaturation No. of

Carbon Atoms None One (Mono- 'Iwo (Diolefines and (Paratfins) olefins) Acetylenes) Ethane. Ethylene Acetylene.

Propane... Propylene Methyl-acetylene. Butzmcs... Butenes gutiidene and Butynes. Pentencs en a lane Pcntanesfl Isoprene.

{Hahn butenes {Dimethyl-ecctylcnc, etc.

However, the above definition should not be regarded as limiting the invention to these specific separations, for it is possible to separate by the process of the invention, hydrocarbons of the same degree of unsaturation, such as l-butene from Z-butene, for example, in which pair the 2-butene behaves as though it were more saturated than the l-butene. Materials of three degrees of unsaturation, such as vinyl acetylene, for example, and cyclic hydrocarbons such as cyclopropane, or cyclobutane, etc. may also be separated from other hydrocarbons of similar boiling point.

The process of the invention is conveniently carried out in an extraction column of conventional design. If desired, the column may be packed with materials to provide improved gasliquid contact, e. g., Raschig rings, helices, plates, etc. According to a preferred mode of operation, the gaseous feed comprising mixed hydrocarbons is introduced into the column at a point near the middle of its length, while the selective solvent is introduced near the top. The raflinate gas, which contains a larger proportionof the less unsaturated constituents than the feed stock, rises through the upper part of the column countercurrent to the flow of liquid solvent and leaves the column at the top, while the extract gas, which contains a larger proportion of the more unsaturated constituents than the feed stock, is absorbed by the solvent and carried down to the bottom of the column where it is discharged. The solvent containing the extract gas taken from the bottom of the column is introduced into a distillation column, which may be equipped with a reboiler and reflux cooler, and by the usual process of fractional distillation the extract gas is separated from the solvent and leaves the columngatthe'atop: Part ofrtheiextr-act gas:

is retumedr-toythe base of the extraction column s to be subjected to further extraction with the solvent, and the remainder passes to storage, The bottoms from the distillationmolumnane,..

passed through a cooler, purified ifnecessary; and recirculated to the extraction;column.-;-,

In a modification of the aboveprocess-,1theire turn to the extraction colummofcsaportioniof the extract gas is accomplished within the extraction column itself by the addition-of; heatlatithec base of the column so as to boil ofi a portion y-of v the extract gas from the solvent. Although these processes are preferred, other modificationsof the solvent extraction process in which the selecture solvents 10f FthQ ;inventionare employed,- are those of the extractive distillation- -type such as; is described. S.- Patent llor,l,9 l8,'7-77; 0r.- ofpther convent-ionaletypessused in liquid; ex-- traction of gaseousnmixturess:-

Th allowingexamulewill illustrate the pram tice o the invention-according;;to--apreferred mode ,of. operation;- but is;.-not, to be "construed limiting the same.;.-

Example I A mixture containing,papproximately equal parts -byweight of butadiene -and-normalbutane is fed at a rate -,of,about 1.8,pounds per, hour into;

the middle-eta packed column about-forty feet highfand three inches in diameter. A solvent consisting ofabout .SO percent by Weight of dioxane and about, ;}per cent by weight of formamide is fed into'tl re top of-thenc0iurnn at a ratchet-about:tilhpounds per hour. A pressure of about 20 pounds gage ismaintained within the column, and the temperature at the bottom of' the -coluinnis maintained at about 180"F by -means-of -a heater. The-remainderofthe column-operates at atemperature of about 100 i The solvent withdrawnfrom the bottom of the extraction columnispassed to a distillation columnwhere the extract' gas is separated from tureswof butenesr. and nibutadiene; butanes and The same process and butenes=,-.and other hydrocarbon mixturesxas indicated prev-iously.

Th mixture oigdioxane .andformamide :used

in theprocessl just, described-is a preferred sol-v vent ior use: according to the invention; How -i.

even;- as,-., previously -mentioned, any ring-saturated. heterocyclic: organic compound vcontaining two OXyZgGl'l BitlOgEI-I'OI' sulfur hetero atoms may be employed insteadof,thedioxane, .All of such compounds display some solvent selectivity for hydrocarbons, but. not to a degree votteohnical importance; However, when much butadiene as butane at F. Thus the addition of formamide to dioxane substantially trebles the solvent selectivity of dioxane for butadiene with respect to butane. Similar increases in solvent selectivity may be effected by the use of, dormamide in admixture: witlr the; other solvents ofthe present classiw The heterocyclic compounds which are employed in admixture with formamide in separating mixtures of gaseous hydrocarbons according to'the invention are preferably liquids under normal;cqnditionsiofstemperature and pressure, althoug'lrjowsmelting solids which are liquids understheiconditionssofz the extraction process may also be employed- A preferred class of heterocyclic com-ponndsis formed of the normally liquid ringesaturated heterocyclic compounds having; two oxygen, sulfur or nitrogen hetero atoms.

'Suchicompounds may be represented by the wherein}; andgYeach represents an oxygen .or

sulfm'-atom;.'or, an NZ group, Z"being,hydrogen -NH2; I r s; I and oneof'the hydrogen atoms maybe replaced. 1''

by a methyligroupg Xand Y may, also be in" the 1-3 instead "Of'fthfi 14 position inthe: 6-mem .-i bered-rin'gg Itwillbe' noted that-all of such com-,1

compounds are; dioxane, methyl 'dioxane; di-

morpholine, methyl morpholine'e-tc.

The proportion of formamide employed; in ad;- mixture with such-heterocyclic solvents may be poundsare ring saturatedgi. e., there; are double bonds in *the 1 ring; Examples :of such" varied bet-Ween relatively wide limits depending upon the nature of the heterocyclic solvent; the

nature-of-=the hydrocarbon mixture to be extracted, ancl-.---i;he conditions under which-the extractioniscarriedout. In-anycase,- =however; it will be an amount suff cient to efiecta sub;

stantial :ilml ease in the-solvent power oi-the heterocyclic solvent for -one:.=ofthe components 1 of the hydrocarbon mixture subjected to the ex tractioneprocessc- Such amount usually repreweight.v ofzthe entire mixed solvent;

Thefollowingexamples are further illustra-r tive of thespractiso of:theinventions Example II admixed with formamideas herein disclosed theirselectivity is greatly enhanced, Thus, for-example, at a tem; perature 'of about-tfli -Fu puredioxa-ne dissolves only about 1.3 times as -much butadiene as butane;

A mixturezofs about 80 :per centby weight of-disentsbe'tween about 5 "and-about -35 percent' by A qmixture :OLJapprQXimateIy"equal parts" by weightg of normal butanewand blltQIlErQ is passed at;-a rate ofvabout,2.5 pounds :per hour upwardly throughna packed/columnncountercurrent to a l0-per cent by; weight. of formarnide. I The sol Venta-issuppl-iedp to the top 0f the column at a ratepofsabout-AOO pounds per hour. The column however, :W ll dissolve more ithanithre itimes as. unis operatedzunderra pressure'onabout" 15 poundsgage and a temperature of about 90 C. The solvent solution of extract gas is fractionally distilled to recover an extract gas containing better than 80 per cent by weight of butene-Z.

Example III A mixture consisting of about 65 per cent by weight of ethylene and about 35 per cent by weight of ethane is extracted with a solvent consisting of about 70 per cent by weight of pyridine and about 39 per cent by Weight of formamide as in Example II. The extract gas analyzes better than 85 per cent by weight of ethylene.

In general, any hydrocarbon gases showing ap preciable differences in solubility in the solvents of the present class may be separated by the process of the invention, the ease and completion of the separation increasing with increasing differences in solubility with the more soluble gas being concentrated in the extract phase. The process is especially well adapted to use in separating hydrocarbons of different degrees of unsaturation, particularly the hydrocarbons containing four carbon atoms.

Many modifications may be applied to the processes described above. For example, the extraction equipment may be fitted with heaters or coolers to provide for extraction at any temperature between the approximate condensation point of the gaseous feed stock and the boiling point of the selective solvent, under the conditions of use. Pressure may vary correspondingly. Flow rates may be varied over wide limits. A temperature gradient may be maintained along the extraction column if desired, in which case the temperature should increase in the direction of fiow of the extract gas. Absorbers may be provided to purify the raffinate and extract gases of residual traces of solvent. In case the composition of the selective solvent is altered in the process of removing the extract gas from it, it may be adjusted by purification or by addition of a required component. The distillation of the extract phase may be conducted in the presence of steam, pressure or vacuum. In general, any of the techniques applied to the process of selective solvent extraction may be applied to the present process.

Other modes of applying the principle of our invention may be employed instead of those explained, change being made as regards the methods or materials herein disclosed, provided the step or steps stated by any of the appended claims, or the equivalent of any such step or steps, be employed.

This application is a continuation-in-part of our copending application, Serial No. 449,194, filed June 30, 1942, now abandoned.

We, therefore, particularly point out and distinctly claim as our invention:

1. A process for the separation of a normally gaseous hydrocarbon mixture which comprises contacting said mixture with a selective solvent comprising a major proportion of a liquid ringsaturated heterocyclic compound containing two hetero atoms selected from the class consisting of oxygen, sulfur and nitrogen atoms and a minor proportion of formamide, thereby dissolving one component of said hydrocarbon mixture Without dissolving another component thereof.

2. A process for the separation of a normally gaseous hydrocarbon mixture with a selective solvent comprising from about 65 to about per cent by weight of a liquid ring-saturated heterocyclic compound containing two hetero atoms selected from the class consisting of oxygen, sulfur and nitrogen atoms and from about 35 to about 5 per cent by weight of formamide, thereby dissolving one component of said hydrocarbon mixture without dissolving another component thereof.

3. The process of claim 1 in which the hydrocarbon mixture comprises hydrocarbons having four carbon atoms.

4. The process of claim 1 in which the heterocyclic compound is dioxane.

5. The process of claim 4 in which the hydrocarbon mixture comprises hydrocarbons having four carbon atoms.

6. A process for the separation of a mixture of butadiene and butene which comprises contacting said mixture with a selective solvent comprising from about 65 to about 95 per cent by weight of a liquid ring-saturated heterocyclic compound containing two hetero atoms selected from the class consisting of oxygen, sulfur and nitrogen atoms and from about 35 to about 5 per cent by weight of formamide, thereby selectively dissolving the butadiene in said solvent, and thereafter separating the butadiene from said solvent.

7. A process for the separation of a normally gaseous hydrocarbon mixture which comprises continuously subjecting said mixture to countercurrent solvent extraction with a selective solvent comprising from about 65 to about 95 per cent by weight of a liquid ring-saturated heterocyclic compound containing two hetero atoms selected from the class consisting of oxygen, sulfur and nitrogen atoms and from about 35 to about 5 per cent by weight of formamide, continuously separating from the extract the component of said hydrocarbon mixture which is selectively dissolved by said solvent, and thereafter returning substantially hydrocarbon-free selective solvent to the extraction.

8. The process of claim '7 in which the heterocyclic compound -is dioxane.

9. The process of claim 7 in which the hydrocarbon mixture comprises hydrocarbons containing four carbon atoms.

ORVlLLE L. POLLY. CLYDE H. O. BERG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,063,680 Isham Dec. 8, 1936 2,128,958 Mueller-Cunradi et al. Sept. 6, 1938 2,366,360 Semon Jan. 2, 1945 2,366,361 Semon et a1 Jan. 2, 1945 2,374,984 Evans et a1. May 1, 1945 2,383,547 Hamill et al. Aug. 28, 1945 

1. A PROCESS FOR THE SEPARATION OF A NORMALLY GASEOUS HYDROCARBON MIXTURE WHICH COMPRISES CONTACTING SAID MIXTURE WITH A SELECTIVE SOLVENT COMPRISING A MAJOR PROPORTION OF A LIQUID RINGSATURATED HETEROCYCLIC COMPOUND CONTAINING TWO HETERO ATOMS SELECTED FROM THE CLASS CONSISTING OF OXYGEN, SULFUR AND NITROGEN ATOMS AND A MINOR PROPORTION OF FORMAMIDE, THEREBY DISSOLVING ONE COMPONENT OF SAID HYDROCARBON MIXTURE WITHOUT DISSOLVING ANOTHER COMPONENT THEREOF. 